1. Field of the Invention
The invention relates to a wavelength tuning device and a wavelength tuning method for tuning a reflected wavelength of a fiber Bragg grating. More particularly, the invention relates to a wavelength tuning device and a wavelength tuning method for automatically controlling variation of the reflected wavelength caused by temperature fluctuation.
2. Description of the Related Art
In recent years, there has been a rapid increase in demand for communication as a result of the increasing popularity of the internet etc., and high speed, large capacity networks using optical fiber are being installed. In high speed, large capacity optical networks of this type, Wavelength Division Multiplexing (hereinafter simply referred to as “WDM”) transmission techniques are indispensable. In particular, great attention is being focused on Dense WDM (hereinafter simply referred to as “DWDM”) techniques in which the wavelength interval is narrowed, and the optical carrier wave is densely arranged on the wavelength axis.
In addition, Optical Code Division Multiplexing (hereinafter simply referred to as “OCDM”) transmission techniques are also gaining attention, and are expected to increase wavelength utilization through combined use with WDM or DWDM, and offer superior transmission security and demultiplexing. OCDM techniques perform demultiplexing by modulating using a different code for each channel on the transmission side, and decoding at the receiving side using the same code as the transmission side. Various techniques are known including techniques that combine time-spread/wavelength-hopping techniques (hereinafter simply referred to as “wavelength-hopping techniques”) in which the assigned sequence on multiple wavelengths and the time domain of each wavelength is encoded, and phase coding techniques in which dispersed optical pulse trains are encoded.
Technology is known in which a fiber Bragg grating (hereinafter referred to simply as “FBG”) is used as the optical filter device in WDM or DWDM systems, or the encoder/decoder in OCDM systems. The FBG is a device that includes a refractive index modulating region with a lattice shape, namely, a grating, formed in the core of an optical fiber. The FBG reflects a specific wavelength. However, it is known that the reflected wavelength of the FBG varies substantially in accordance with the external temperature (hereinafter also referred to as the “ambient temperature”) surrounding the FBG as a result of temperature dependence of the refractive index of the FBG and expansion/contraction of the optical fiber caused by temperature, etc.
Methods for inhibiting variations of the reflected wavelength of an FBG caused by ambient temperature have been proposed such as that disclosed in Published Japanese Translation of PCT Application, JP-T-2000-503415. According to the method disclosed in JP-T-2000-503415, an optical fiber in which an FBG is formed is fixed on a ceramic board that has a negative coefficient of thermal expansion. Variations in the reflected wavelength of the FBG caused by fluctuations in the ambient temperature are corrected based on a change amount of the reflected wavelength of the FBG caused by expansion/contraction of the optical fiber, in accordance with expansion/contraction of the ceramic board.
However, particularly in the case when the FBG is used as the OCDM encoder/decoder described above, if there is any deviation in the reflected wavelengths of the FBGs between each paired encoder and decoder, transmission quality deteriorates. Accordingly, the reflected wavelengths must be made substantially the same. However, in the above method, wavelength variation can occur that can cause a wavelength difference between the encoder and decoder that exceeds the level of wavelength difference that is necessary to maintain transmission quality. Moreover, in the above method, the reflected wavelength is determined at the stage when the optical fiber is fixed and cannot be changed. Thus, if the optical light source wavelength varies, it is not possible to adjust the reflected wavelength of the FBG, and transmission quality deteriorates.